In this project we will continue to study the morphogenesis, structure and function of the murine mammary tumor virus (MuMTV) and its components, as well as their interaction with the host cell. We will study the degree of phosphorylation of the MuMTV gag precursor polyprotein contained in A particles, its primary structure, and the nature of the polypeptide sequences exposed on the surface of A particles from the MuMTV producer and non producer cells in order to understand the structural requirements of A particles for budding. Somatic cell fusion of defective MuMTV producer and MuMTV non producer A particle-containing cells with cells that permit productive MuMTV infection will be done in order to determine if cell hybrids produce complete MuMTV. This experiment will enable us to determine the possible role of a specific host cell factor(s) in MuMTV morphogenesis. We will also purify and characterize the enzyme activity in mature MuMTV that cleaves the in vitro synthesized gag precursor polyprotein, in order to achieve a better understanding of the maturation process of MuMTV. We will study the possible functional role(s) of the two MuMTV phosphoproteins by analyzing their oncoviral RNA binding specificities, and by determining the effect of their exogenous addition on the reverse transcription of MuMTV genomic RNA, using purified MuMTV DNA polymerase, in a reconstituted system. We have previously found that the MuMTV env precursor is shed from virus-producing tissue culture cells of tumor origin. We will therefore determine if the circulating viral antigen in mammary tumor bearing mice is the env precursor, and will also examine the possibility that shedding of MuMTV env precursor might be a marker for cell transformation. We propose to isolate the MuMTV gp52-gp36 complex from virions in native configuration and test the effectiveness of this complex as a vaccine against mammary tumor development.